A gas dosimeter of this kind is disclosed in United Kingdom Pat. No. 2 084 725 B.
A strip impregnated with a detection reagent, when exposed to the gas that is to be detected, develops a color change which with sustained exposure time progresses along the strip. The colored length then represents the measure for the gas concentration that is to be detected.
Especially when measuring low concentrations of a toxic substance over a relatively long measuring period, such reagents, which provide a sensitive and specific color reaction with the gas to be detected but in which the pigment formed is soluble within the impregnation at a high diffusion rate, cannot be used as a length indicator, because instead of remaining as an indicator agent in the reaction zone, the pigment distributes out of the reaction zone into the zones where a reaction has not yet occurred, making a length reading impossible. If required, an evaluation would be possible with respect to the color saturation, but without auxiliary aids this is even more difficult and less accurate than a length reading.